Hook into Zygote.jl?

I want to take gradient of a vectorized expression, however the adjoint of vmaterialize is not defined.
```
using Zygote
using LoopVectorization
foo(x) = sum(@avx tanh.(x))
x = rand(10, 10)
foo'(x)
```
```
MethodError: no method matching vmaterialize(::Array{Float64,2}, ::Val{:Main})
Closest candidates are:
vmaterialize(::Base.Broadcast.Broadcasted, ::Val{Mod}) where Mod at /BIGDATA1/highchain_ylu_1/shared/.julia/packages/LoopVectorization/MZ8Kb/src/broadcast.jl:283
```

Start by reproducing the provided Zygote and LoopVectorization example, then inspect vmaterialize in src/broadcast.jl around line 283. Trace how the vectorized broadcast is represented during differentiation. Done means the shown foo'(x) call works without the missing-adjoint MethodError and the relevant behavior is covered by a test.

Vom Indexierungsmodell aus dem Issue-Text verfasst.